In interstitial radiation therapy, a tumor can be treated by temporarily or permanently placing small, radioactive seeds into or adjacent the tumor site. This can be accomplished by implanting loose seeds in the target tissue, or by implanting in the target tissue seeds that are connected to one another by a bio-absorbable material.
To implant loose seeds, an applicator device (e.g., a MICK® applicator or the like) that includes a needle is often used. A stylet is initially fully extended through a bore in the needle and the needle is inserted into a patient in an area where a row of loose seeds are to be implanted. The stylet is then retracted from the needle, enabling a loose seed from a magazine to enter the bore of the needle. The stylet is then pushed against the loose seed, forcing the seed through the bore of needle and into the target tissue. After a first seed has been implanted, the needle is withdrawn from the patient's body by a particular distance so that a next seed to be implanted is spaced apart from the first seed. Then, the stylet is again retracted to enable the next seed from the magazine to be positioned for movement into the needle. The stylet is then advanced through the needle to force the next seed into the target tissue at a desired distance away from the first seed. This procedure is repeated for subsequent seed implants. Additional details of this implantation technique and the applicator used to perform this technique can be found in U.S. Pat. No. 5,860,909, which is incorporated herein by reference.
In the above technique, loose seeds are deposited in a track made by the needle. However, when the needle is withdrawn, there is a tendency for the seeds to migrate in that track resulting in improper distribution of the seeds. Additionally, after implantation, the loose seeds are dependent on the tissue itself to hold each individual seed in place. This may result in the loose seeds migrating over time away from the initial site of implantation. Such migration of seeds is undesirable from a clinical perspective, as this may lead to underdosing or overdosing of a tumor or other diseased tissue and/or exposure of healthy tissue to radiation. The loose seeds may also rotate or twist from the original orientation at which the seeds were implanted. This is also undesirable from a clinical perspective, because the radiation pattern of the seeds may be directional, thereby causing underdosing or overdosing of a tumor or other diseased tissue and/or exposure of healthy tissue to radiation. Further complicating the implantation of loose seeds is the fact that the seeds are small, because they need to fit in small bore needles to prevent excessive tissue damage. Due to their small size and high seed surface dose, the seeds are difficult to handle and to label, and can easily be lost. In addition, the above described technique for implantation of individual loose seeds is time consuming.
Because of the disadvantages of using loose seeds, many physicians prefer using elongated members (often referred to as strands) that contains multiple seeds spaced from one another at desired increments. Such strands are capable of being loaded into an introducer needle just prior to the implant procedure, or they may be pre-loaded into a needle. Implantation of strands is less time consuming than implanting loose seeds. Additionally, because the seeds in the strands are connected to one another by a bio-absorbable material, there is less of a tendency for the seeds to migrate and/or rotate after implantation.
There are numerous techniques for making strands that include multiple seeds. For example, such strands can be made using a bio-absorbable material, with the seeds and rigid teflon spacers between the seeds inserted into the material. Needles loaded with the seeds in the carrier bio-absorbable material are sterilized or autoclaved causing contraction of the carrier material and resulting in a rigid column of seeds and spacers. This technique was reported in “Ultrasonically Guided Transperineal Seed Implantation of the Prostate: Modification of the Technique and Qualitative Assessment of Implants” by Van't Riet, et al., International Journal of Radiation Oncology, Biology and Physics, Vol. 24, No. 3, pp. 555-558, 1992, which is incorporated herein by reference. Such rigid implants have many drawbacks, including not having the ability to flex with the tissue over the time that the bio-absorbable material dissolves. More specifically, as the tissue or glands shrink back to pre-operative size, and thus as the tissue recedes, a rigid elongated implant does not move with the tissue, but remain stationary relative to the patient. The final locations of the seeds relative to the tumor are thus not maintained and the dosage of the radioactive seeds does not meet the preoperative therapy plan. Accordingly, there is a desire to provide a strand of seeds that is capable of moving with tissue or glands as they shrink back to pre-operative size, thereby enabling the seeds to meet a preoperative therapy plan.
In another technique, disclosed in U.S. Pat. No. 5,460,592, which is incorporated herein by reference, seeds are held in a woven or braided bio-absorbable carrier such as a braided suture. The carrier with the seeds laced therein is then secured in place to form a suitable implant. This braided assembly exhibits many drawbacks, as and when the braided assembly is placed into the target tissue. The needle that carries the braided strand assembly must be blocked at the distal end to prevent body fluids from entering the lumen. If body fluid reaches the braided strand assembly while the assembly is still in the lumen of the needle, the braided assembly can swell and jam in the lumen. Because the assembly is made of a braided tubular material, it is difficult to push the assembly out of the needle. As the needle is withdrawn from the tumor, pressure on the proximal end of the braided strand assembly causes the braid to expand and jam inside the lumen of the needle. Finally, if the braided strand is successfully expelled from the needle, the relative spacing of the seeds may not be maintained, if the braided material has collapsed. Accordingly, there is also a desire to provide a strand of seeds that can be implanted without causing jamming of a needle, and that after implantation the strand maintain the desired spacing of the seeds.
It is also desirable for a strand of seeds to be echogenic, i.e., be visible using ultrasound imaging, so that the implant can be visualized during implantation and during post operative visits to a physician. Techniques have been developed for making the seeds themselves more echogenic. For example, U.S. Pat. No. 6,632,176 suggests that seeds can be roughened, shaped or otherwise treated to improve the ultrasound visibility of the seeds. However, it is desirable that an entire strand be visible, not just the seeds therein. It has been suggested that the particles of materials such as glass, silica, sand, clay, etc. be mixed in with the bio-absorbable material to make the strand assembly of seeds more visible to ultrasound. However, the additions of such particles may effect the integrity of the strand. Additionally, such particles may irritate tissue after the bio-absorbable material has been absorbed. Further, it may be desirable to simply minimize the volume of materials that are not going to be absorbed by the body. Also, because it may be difficult to control the distribution of such particle, strand including such particles may not be uniformly visible by ultrasound.
Another technique that has been suggested to increase the ultrasound visibility of a strand of seeds is to introduce air bubbles into the bio-absorbable material during the manufacture of the strand, since air is a strong reflector of ultrasound energy having an inherent impedance many times greater than body tissue. This can be accomplished during the cooling stage of a molding process used to produce the strand, as disclosed in U.S. patent application Ser. No. 10/035,083, filed May 8, 2003, which is incorporated herein by reference. More specifically, during the cooling stage, the mold is placed in a vacuum chamber and the air in the chamber is evacuated. This causes the entrapped air in the mold to come out of solution from the polymer, and as the mold cools, this air is entrapped within the cooling polymer in the form of minute bubbles suspended in the plastic. A potential problem with this technique, however, is the inability to control the placement and size of the air bubbles. Thus, a strand including such air bubbles may not be uniformly visible by ultrasound. Accordingly, there is also a desire to improve the ultrasound visibility of a strand of seeds.